Asynaptic effect of chromosome V
M. OKAMOTO
Curtis Hall, University of Missouri, Columbia, Missouri, U.S.A.
It is known that chromosome III of the Chinese Spring variety has a marked
effect on chromosome pairing, and that chromosome II of the same variety
has a similar but less pronounced effect (Sears 1944). The other chromosomes
of Chinese Spring have not been suspected of any effect on chromosome
pairing.
In the F1 between plants which were monosomic for a telocentric
chromosome V and AADD plants (amphidiploid T. aegilopoides x Ae.
squarrosa), 34-chromosome plants which did not carry the telocentric
chromosome showed unexpectedly much better pairing than 35-chromosome
plants, as is shown in the following table of data from typical plants
:
If chromosome V belongs to the A genome (Larson 1953), the 34-chromosome
plants would be expected to give more univalents and fewer bivalents than
35-chromosome plants. But the above table shows that the average number
of univalents is much less and that of bivalents is much more in 34-chromosome
plants than in 35-chromosome plants:
If chromosome V belongs to the B genome, 34-chromosome plants would be
expected to give fewer univalents than 35-chromosome plants, and the number
of bivalents would remain the same. This very slight increase in the expected
frequency of univalents comes nowhere near explaining the observed increase
of 15 per cell.
A possible explanation for the above facts is that the telocentric chromosome
V and hence chromosome V of Chinese Spring carries a gene or genes for
asynapsis. Then it follows that chromosome pairing is good in 34-chromosome
plants due to the absence of the asynaptic effect of chromosome V, while
35-chromosome plants show poor chromosome pairing due to the asynaptic
effect of the telocentric chromosome V. This asynaptic effect of chromosome
V has only been observed in this pentaploid hybrid, where two sets of
each of the A and D genomes and one set of the B genome are present.
(Received March, 16. 1957)